1. Field of Invention
The present invention relates to the wet-processing systems used in microelectronic fabrication. More particularly, the present invention relates to the systems used for cleaning after microelectronic device manufacturing process steps such as the pre-diffusion cleaning, wet etch processes, photoresist removals and other cleaning operations.
2. Description of the Related Art
Particle contamination is an important source of semiconductor chip yield losses. As chip geometry becomes smaller, even very small particles, such as those less than 0.1 microns in diameter, can cause defects that decrease yield and product reliability. Despite the importance of this problem, major gaps exist in industry knowledge and technology. This problem is especially acute for the wet processes and rime systems.
The microelectronic devices are typically processed using chemical solutions and rinses. For example, a semiconductor wafer is typically made from silicon with various materials implanted or diffused into or applied on the surface. During a wet etch step in the process, a group of wafers to be etched are placed in a container called a boat that is placed in a chemical solution. After this, the boat is removed from the chemical solution and rinsed with deionized (DI) water to stop the etch. The rinse can also help remove particles that happen to adhere to the wafer. Unfortunately, the water used in rinsing process can introduce particles that adhere to the wafer. Handbook of Quality Integrated Circuit Manufacturing by Robert Zorich, ISBN 012-781870-7 provides a description of circuit manufacturing techniques and concerns, and the entire disclosure is hereby incorporated by reference.
Some operations in semiconductor manufacture require that there be no oxide layer on the wafer. However, if any particles contact the bare wafer after bathing in hydrofluoric acid (HF) before a protective oxide layer is formed, the particles may strongly adhere to the wafer. Further, a thin protective oxide layer forming whenever bare silicon is exposed to oxygen may protect against further reactions and contaminants but will not always reduce particle contamination. For example, when the oxide layer is formed on the wafer from contact with air, water may not effectively rinse the particles from the surface.
Techniques for Wafer Cleaning
Certain techniques for removing particles from wafers have major disadvantages. Wet chemical cleaning, for example, may add particles. Scrubbing is unsuitable for the removal of submicron particles, and can damage wafers. Pressurized fluid jets may damage wafers and lead to electrostatic charge build up that can also damage wafers. Ultrasonic cleaning can damage wafers, does not allow the use of reactive solutions, and makes it difficult to control cavitation. Megasonics may introduce contamination from chemical solutions. Strippable polymer may leave a deposit of polymeric residue and has not been demonstrated to be effective. Using UV light with ozone has not been proven effective for particle removal and can damage the wafer.
There are two general techniques for rinsing wafers both of which involve placing a boat into a tank of DI water. Cascade rinsing uses a continuous flow of DI water that spills, or cascades, over the top of the tank. Quick-dumping rinsing evacuates the water from the tub as quickly as possible.
Sources of Contaminants During Rinsing
Particles contaminating the wafer during the rinsing process come from the rinse water and air. Waterborne particles can be introduced from the pipes of a DI water distribution system. In a semiconductor foundry, DI water is typically purified from the local water supply at a central facility and then supplied to wet benches throughout the foundry by the distribution network. Even if pure water leaves the central facility, bacteria growing in the pipes can enter the rinsing system, and contaminate the wafers. In addition, the inner surface of the piping itself can introduce particles.
Operating a clean air and water distribution network throughout the foundry presents an economic problem. Different parts of the manufacturing process place different demands on the cleanliness of the air and water. For example, post-HF wet processing requires extremely clean air and water, while other operations may be less susceptible to particle contamination. The cost of operating a cleaning facility is related to the degree of cleanliness and the volume of water required. Using only a centralized purification system requires that all of the air and water purified for all of the foundry that meet the requirements of the most demanding operation. Because only a relatively small amount of air and water needs to be of the highest purity, this may result in an unduly expensive central water purification system.
To help eliminate contaminants that develop in the water distribution system relying on centralized purification, the entire system may be purged. For example, an H.sub.2 O.sub.2 purge and DI water flush can remove bacteria growing in a DI water system. In such a centralized system, however, wafers cannot be produced while the system is being purged, causing production delays and shutdown of some operations. Consequently, such steps may be taken only when contamination problems become acute. This exposes the wafers to significant levels of impurities in the water between the system purges.
Airborne Contaminants
Rinse systems also expose the wafers to airborne contaminants when the wafers are exposed to air in being transferred to the rinse tank. One way of processing wafers involves putting wafers in a full tank, and then repeatedly emptying and filling the tank with DI water. When the wafers start the emptying-filling process, they are immersed in water. In quick dumping systems, room air displaces water as the water leaves the tank. This exposes the wafers to airborne contaminants. As the tank refills with water, the DI water displaces the air surrounding the wafers.
Rinse systems are also prone to contamination arising from bubbles that develop when the DI water enters the tank in a rinsing system. Very small bubbles are referred to as micro bubbles. Bubbles arise from turbulence and pressure changes that occur where a pipe or channel carrying the water, changes dimensions or mixes with air. The bubbles may trap particles which can then be deposited on the wafer surface if the bubble contacts the wafer surface.
Dissolved Oxygen
Dissolved oxygen in the rinse water may also present problems. Dissolved oxygen affects the corrosion rate of metallic materials contacting the DI water. Ion exchange resins are also oxidized by dissolved oxygen which degrades the resin and shortens its life. In addition, the growth of bacteria in pipes and equipment is accelerated by dissolved oxygen. Accordingly, foundries are reducing the amount of dissolved oxygen in their DI water. However, removing the dissolved oxygen can impede the formation of a native oxide film on the wafer.
Particle Protection Techniques
Wafers are exposed to airborne contaminants at different stages in the process. One method to protect wafers from airborne contaminants is to put the wafers in special enclosures during transport between process steps. However, this does not solve the problem of impurities in the DI water, chemicals and air in the rinse equipment. Another method of minimizing contamination involves the use of isopropyl alcohol (IPA) vapors for cleaning and drying the wafers during and after the rinse step. This is environmentally undesirable and expensive.
In some applications, it is useful to clean the wafer with a solution containing one or more cleaning agents. One method of doing this is to bathe the wafer in one tank with a cleaning agent, and then move the wafer to a different tank for rinsing. This requires having multiple tanks and exposes the wafer to contaminants while being transported from one tank to another.